Transmission of electrical and optical signals may be effectively used to communicate information. Microwave signals are electrical emissions transmitted through the air to carry audio and visual information over distance. Similarly, electrical signals may be transmitted over cables, as is done in telephone systems and cable television systems.
Recently, in communications systems, such as telephone systems, emphasis has turned toward use of optical signals and away from use of electrical signals. In modern times, telephone companies implement optical fiber communication systems. In the home electronics industry phonographs have been replaced with compact disk players, which rely upon laser light reflections to read information from a disk.
Practical reasons exist for the shift in focus to optically driven systems. Unlike electrical signals, optical signals are generally unaffected by electromagnetic fields created by such things as power lines, lightning and even sunspots. These sources of interference may create noise in electrical signals. Noise may appear, for instance, as static in an audio signal or distortion in a visual signal. Thus, while such electromagnetic fields create noise in a electrical communication system, an optical system retains its original qualities in the presence of the electromagnetic fields.
Information capacity of optical signals is also much larger than lower frequency electrical signals that are used in wire and wireless communication systems. Generally, higher frequency signal carriers provide larger information capacity than lower frequency signal carriers. This is due to the wider bandwidth of the higher frequency signals.
Larger information capacity and noise immunity are great benefits, but another important benefit of communicating with optical signals is the small size of optical fibers used as a transmission medium. A typical fiber having hair sized dimensions is a suitable replacement for bundles of copper wires having much larger diameter. As demands for information access become larger and larger in modern times, the use of optical transmission systems places less demand on space in the construction of underground, above ground, and internal building communication systems.
Common difficulties are encountered in the practical implementation of optical communication systems, however. Ideally, the basic elements of a communication system include a transmitter, a transmission medium, and a receiver. Input signals, typically electric signals, are input to an optical transmitter. Conversion of the input signal to an optical signal is conducted within the transmitter and a light source, such as a semiconductor laser, pumps light into the optical transmission medium. The transmission medium usually takes the form of an optical fiber. Reception and conversion of the optical signal is accomplished in a receiver coupled to the optical fiber at some distance away from the transmitter. A basic receiver will include a light detector for detecting the optical signal and converting the same to an electrical signal, an amplifier for amplifying the electrical signal, and signal reproducer for outputting the original input signal as an electrical signal.
In practice, additional elements are required since signal losses occur over distance in the optical fiber. Losses limit the distance by which the transmitter and receiver may be separated. These losses are generally referred to as optical signal attenuation. Absorption of signal light by the fiber acting as the transmission medium is one factor causing attenuation. Other factors leading to attenuation are the scattering of the signal light over a wider wavelength than the original transmission and radiative losses, typically occurring at bends in the optical fiber. Combination of these individual losses leads to a total signal attenuation characteristic for a particular optical transmission medium which is measured in decibels per kilometer.
In order to implement practical systems, taking into consideration the optical attenuation characteristic of the particular optical fiber being used, it is therefore necessary to periodically amplify the signal as it travels over distance. Repeater stations are used to accomplish this amplification and are an integral part of modern optical communication systems. Typical repeater stations include both a receiver and transmitter which decode the optical signal, convert it to an electrical signal, reconvert to an optical signal and transmit the optical signal toward the next repeater or receiver station.
Repeater stations contribute significantly to the cost of optical communication systems, commonly costing tens of thousands of dollars. Moreover, repeater stations are provided in redundant pairs or larger numbers of repeaters, since a repeater may fail. Additionally, the repeater stations are often installed in inconvenient locations, such as the ocean floor, that makes replacement and initial installation difficult and expensive.
A simpler manner of implementing repeater stations involves use of optical amplifiers. The general structure of an optical amplifier is detailed in U.S. Pat. No. 5,309,452 to Ohishi et al., which is hereby incorporated by reference. In an optical amplifier, the signal light is amplified in optical form without conversion to an electrical signal. Amplification is accomplished by stimulating the signal with additional photoluminescence as it passes through the optical amplifier. Of course, the optical amplifier has other applications, including implementation at the transmission end of an optical communication system to create stronger optical signals that may travel further in a fiber having given attenuation characteristics.
Additional photoluminescence is attributable to what is commonly referred to as a pumping mechanism. Signal light of a given frequency enters the optical amplifier, which is composed of glass or other transmissive material. Glass in the amplifier is also subjected to pumping light from an excitation source which the glass absorbs and which stimulates additional photoluminescence emissions in the amplifier that are imparted to the optical signal. Gain is realized when excitation source light is absorbed and the resulting photoluminescence emissions from the glass coincide with the wavelength of the optical signal.
Traditionally, the optical amplifiers and transmission mediums have been formed with oxide glasses. A widely applied amplifier using oxide glass is the Erbium doped fiber amplifier (EDFA). More recently, chalcogenide glasses have been investigated as hosts since these glasses have good infrared wavelength transparency, are durable, are easy to prepare in bulk or thin film form, and may be formed as patterned waveguides by photodarkening processes. The ability to create chalcogenide thin films, by sputtering, for instance, allows for formation of a device using a chalcogenide glass as part of a larger semi-conductor integrated package.
Typical EDFA's rely exclusively upon the pumping absorption and emission characteristics attributable to the dopant, i.e. Erbium (Er). Effective absorption of light from the excitation source by the EDFA requires that the excitation light correspond to narrow characteristic absorption bands of the Erbium dopant. Incident light in these bands will excite electrons of Erbium ions within the glass to higher energy levels, and photons are released to provide luminescence when the electrons return to the normal state. In EDFA's pumping is therefore limited to the narrow absorption peaks corresponding to the Er.sup.3+ energy level transitions, at 810 nm (the .sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.9/2 transition), 980 nm (the .sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.11/2 transition) and 1480 nm (the .sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2). These absorptions result in photoluminescence emissions in a small band near 1550 nm as excited electrons return to a normal state during the .sup.4 I.sub.13/2 .fwdarw..sup.4 I.sub.15/2 radiative transition. For these reasons, precision excitation sources, such as wavelength tailored semiconductor lasers, must be confined to those narrow absorption peaks. Such sources may be expensive and difficult to produce since fabrication techniques and tolerances must insure that the source emit light at the narrow absorption peak. Additionally, use of broader band sources, such as light emitting diodes, is not effective since the majority of excitation light produced falls outside of those absorption peaks.
In an effort to expand the strictly constrained absorption techniques, other dopants may be introduced along with or in place of the Erbium, as discussed in Ohishi et al., U.S. Pat. No. 5,309,452. For instance, the use of other dopants, such as Ytterbium and Praseodymium may modify the absorption characteristic as these dopants also have characteristic absorption transitions. Additionally, chalcogenide and other hosts are also contemplated. However, excitation sources are still tuned to the particular absorption peaks of the utilized dopants, and the aforementioned absorption pumping limitations still apply. In other words, since the narrow peak absorption transitions of the dopants are relied upon, the excitation sources are still subject to the similar constraints as the EDFA's.
Outside of amplification applications, reliance on the absorption transition peaks attributable to the dopants also limits utility. Exemplary is application of a doped glass as an optical detector. Since emissions of a given wavelength occur in response to characteristic absorption transitions of the dopants, monitoring of the particular wavelength will indicate whether or not the glass is subject to incident light of those particular wavelengths. However, the discrete absorption peaks correspond to a small region of detection. Artisans will also recognize the limitations placed upon other applications which rely upon the discrete and narrow absorption peaks of dopants introduced into a glass host.
In sum, there is a need for an optical pumping system having broad band continuous absorption characteristic which allows for great flexibility in the excitation source used while providing good photoluminescence emission characteristics.
It is therefore an object of the present invention to provide an improved optical pumping system that uses an absorption band extending beyond characteristic dopant absorption transitions.
Another object of the present invention is to provide an improved optical pumping system responsive to excitation pumping light over a continuous broad absorption range of approximately 400 nm.
Still another object of the present invention is to provide an improved optical pumping system which can utilize excitation sources, such as light emitting diodes and simple semiconductor lasers, that supply pumping light over a broader spectrum than finely tuned and expensive laser sources.
An additional object of the invention is to provide an improved optical pumping system having a chalcogenide glass doped with a rare earth which absorbs excitation light from an excitation source over an approximate wavelength band of 600-1064 nm.
A further object of the present invention is to provide an improved optical pumping system having a chalcogenide glass and a rare earth dopant in which characteristic absorption peaks of the rare earth dopant are superimposed on a broad absorption band of approximately 400 nm in width.
A still further object of the present invention is to provide an improved optical pumping system having a chalcogenide glass doped with a rare earth responsive to an excitation source, the glass having a broad absorption band and being suitable for thin film deposition in an integrated circuit including the excitation source.
Yet another object of the present invention is to provide an improved optical pumping system having a chalcogenide glass doped with Erbium or Praseodymium, or both, the chalcogenide glass absorbing pumping light from an excitation source over a broad absorption band of approximately 400 nm in width.
A still additional object of the present invention is to provide an improved optical pumping system having a rare earth doped chalcogenide glass in which electrons are excited to a higher energy level in response to excitation source light over a broad absorption range of approximately 400 nm in width, the electrons having higher energy level occupancy lifetimes in the approximate range of 0.25 ms (Praseodymium) to 2 ms (Erbium).